Examples of a method for treating a waste such as a general waste or an industrial waste may include a method with which a waste is melted in an industrial furnace by using a carbon-based solid fuel such as cokes as a melting heat source. The waste treatment by means of melting is advantageous not only in that the volume of wastes can be reduced but also in that incinerated ashes or non-combustible wastes, which previously would have been finally disposed by landfill, can be transformed to slags and metals for recycling.
A known facility for melting wastes is a shaft type gasification melting furnace (see Patent Literature 1 and Patent Literature 2, for example). The waste gasification melting furnaces disclosed in Patent Literatures 1 and 2 each include a furnace main body including a cylindrical shaft portion (an upright barrel portion), an inverted truncated cone portion (a pavilion portion), and a furnace bottom portion. The waste gasification melting furnaces each have an upper-stage tuyere and a lower-stage tuyere for blowing a combustion supporting gas such as air or oxygen-enriched air into the furnace.
According to the waste gasification melting furnaces disclosed in Patent Literatures 1 and 2, wastes and cokes are charged from an upper portion of the furnace and the wastes descending through the shaft portion are heat-exchanged with air blown from the upper-stage tuyere so as to dry and pyrolyze the wastes. The pyrolysis residues of the dried and pyrolyzed wastes descend to the furnace bottom portion to be melted with the combustion heat of the cokes being used as a heat source. Thereafter, the melting residues are drawn from the furnace bottom portion and slags and metals are collected.
According to the waste gasification melting furnaces disclosed in Patent Literatures 1 and 2, wastes are dried and pyrolyzed by air blown through the upper-stage tuyere. Therefore, in order to facilitate the drying and pyrolysis of the wastes, it is preferred to increase a percentage of oxygen to be supplied from the upper-stage tuyere by increasing a blast amount from the upper-stage tuyere, for example. By increasing the percentage of oxygen to be supplied from the upper-stage tuyere, the drying and pyrolysis by means of the combustion heat from the wastes themselves can be facilitated in the shaft portion.
According to the waste gasification melting furnaces disclosed in Patent Literatures 1 and 2, however, a blow-by phenomenon or the like is generated especially if the oxygen percentage from the upper-stage tuyere exceeds the majority, thereby resulting in an unstable operation of the furnace, for example, fluctuations in a pressure in the furnace as described also in the literatures. Thus, when actually operating the furnace, the oxygen percentage from the lower-stage tuyere is set at 70 to 80% and the oxygen percentage from the upper-stage tuyere is kept at 20 to 30%. In addition, the shaft-type gasification melting furnace has a configuration according to which air blown through the upper-stage tuyere is hard to reach a central region of the shaft portion. Therefore, wastes descending through the central region of the shaft portion are insufficiently dried and pyrolyzed, thereby increasing an amount of moisture content or volatile content to be carried over into the furnace bottom portion. As a result, some of the combustion heat from the cokes, serving as a melting heat source, is consumed for evaporating such moisture or volatile. An extra amount of cokes used for that purpose is therefore necessary to be provided. Such excess consumption of cokes leads to not only a problem of driving up the running cost thereof but also a problem of increasing CO2 emissions generated from a fossil fuel.